1. Field of the Invention
The present invention relates to an anti-veiling-glare glass input window for an optical device, in particular suitable to be used as an input window for an image intensifier, and to a method of manufacturing such a window.
2. Detailed Description of the Prior Art
The term "image intensifier" is commonly used in the Art to describe a device for providing a bright picture of a dimly-lit scene. The device gathers radiation--which may be visible light or, perhaps, Infra-Red (IR) radiation--from the scene, and by means usually involving the conversion of the formed radiation image into an equivalent electron image, the electrical amplification of this latter, and the subsequent re-conversion of the amplified electron image into a visible-light image, it provides a much brighter view of the scene than could possibly be obtained by the naked eye.
Over the past twenty years or so the mode of construction and operation of image intensifiers has changed resulting in smaller, more efficient devices. In the earliest intensifiers--what are now called the first generation devices--the incoming radiation was imaged via a fibre-optic faceplate onto a photocathode layer (carried by the other side of the plate), and the streams of generated electrons were accelerated (via the intensifier section) through an electron-focussing field onto a photoanode--a phosphor, or luminescent, screen converting incident electrons to visible light. At a later date, the so-called second-generation intensifiers was developed, in which instead of the extremely complicated and bulky fibre-optic faceplate and electron-optic intensifier sections there was used a simple glass input window (again carrying the photocathode layer) together with what is now known as a "microchannel plate" in which, essentially, the generated electrons riccochet along the individual tubes (the channels) of a bundle of tubes, each tube-wall-contact generating a cascade of electrons so that each single electron at the input of a tube results in many electrons at the output of the tube, the output electrons being directed to a photoanode/luminescent screen to provide the required bright visible-light image. Presently, much work is being done on third-generation devices, which are very similar to the second-generation microchannel plate devices save that they employ thin semiconductor films on a glass cathode window; typical cathode material for the third generation devices are gallium-arsenide, aluminum gallium arsenide and indium gallium arsenide.
Both second- and third-generation intensifiers employ microchannel plates positioned adjacent one side of some electron-generations section (cathode), and on the other side--the input side--of the latter they include a plain glass input window through which the radiation from the viewed scene enters the intensifier. This window conventionally seals the intensifiers internal parts away from the ambient environment, and the inner surface adjacent the electron-generations section is usually employed to support a conductive layer constitutions the electrical connection to that section. Unfortunately, because of the physical shape of the most common types of input window, the conductive layer can cause severe problems arising from the internal reflection at the window/conductive layer boundary of off-axis light into the intensifier proper, so resulting in spurious image formation.
An intensifier input window is generally shaped so as in cross-section to have the shape of what can best be described as a very fat and very short T. The main body of the window is the upright of the T, and it has a circumferential radial flange at one face (the outer face, on the input side) to form the crossbar of the T. Finally, the inner face edge (defined by the bottom corners of the T upright) is bevelled (at about 45 degrees), the bevel extending all the way to the flange.
As mentioned above, the window is employed to support a connecting conductive layer, usually in the form of a vapour-deposited metal. The conductive layer is generally disposed on the bevelled surface and on the adjacent lower surface of the flange. It will be clear that, depending on its precise angle of incidence, light entering the window at an angle to the normal--i.e. from a source off the axis of the intensifier--may well be reflected at the window/conductive layer interface, and continue on into the intensifier to generate a spurious, or ghost, image.
Ghost images from off-axis sources can be very disturbing to the user of the intensifier, and various methods have been tried to eliminate them, or at least to reduce them to an acceptable level. One such method involves the use of input windows of the type known as "bullseye" or "saturn" windows, in which the central portion of the window (the core of the T upright) is made of clear glass, but the rest--the flange (the T crossbar) and the part adjacent the bevelled surface (the outer portion of the T upright)--is made of a dark (usually black) light-absorbent glass. Such a cathode window has been disclosed in U.S. Pat. No. 4,406,973.
A drawback of the known window is, that there still may occur unwanted reflections at the dark glass/clear glass interface resulting in spurious images. Also the known window is difficult to manufacture and hence rather expensive. According to U.S. Pat. No. 4,406,973 the known window is manufactured by taking a mass of clear glass and forming an annular channel in the mass of clear glass. The annular channel is filled with fluid black glass and the resulting disc of clear and black glass is machined to obtain a T-shaped window having a circumferential portion of black glass enclosing a cylindrical core of clear glass.
Another method of manufacturing the known window would be to heat-shrink an appropriate diameter tube of dark glass onto an appropriate diameter rod of clear glass, sawing the obtained rod into a number of discs each having the general dimensions of the required windows, and then shaping the circumferential surface of each disc by machining away the excess dark glass.
In both methods considerable difficulty can be experienced in properly centering the disc ready for the machining stage and frequently glass is removed excentrically, so exposing near the smallest diameter section of the bevelled part the clear glass core, thus leaving part of the resulting window unshielded by black glass. As in a later stage the bevelled surface to the input window will be covered with a metallic conductive layer this will result in unwanted reflections.
Also in both methods there is a high possibility of mechanical fracture and of bubble formation at the black glass/clear glass interface.
Further it is known from PCT patent application WO 84/04821 to manufacture an input window rejecting off-axis radiation by taking a cylindrical core of clear glass and cladding the cylindrical surface of the core with a special kind of glass containing thin fibers or leaves of radiation absorbing glass. The cladding glass is first made as a flat sheet, from which bevelled strips are cut. The bevelled strips are then arranged on the cylindrical surface of the core and fused together.
This known method is rather complicated and hence expensive. Also there will still occur some reflection on the cladding glass/core interface and there is a high possibility of mechanical fracture and bubble formation at the cladding glass/core interface.